This invention relates to planar silicon semiconductor devices, particularly of the bipolar type, which operate at relatively high voltages and include field plates to inhibit surface inversion. Planar devices are described as having a plurality of PN junction boundaries intersecting a common major surface of the semiconductor body.
It is now a well-known procedure to passivate the surface of silicon semiconductor devices by applying a dual dielectric layer to the active surface of the device. Typically, this dielectric layer consists of a first layer of silicon oxide, usually grown on the silicon surface, overlaid by a deposited layer of silicon nitride. The genetic oxide is provided primarily to stabilize the device surfaces electrically, while the layer of silicon nitride provides a barrier against certain persistent active ions, in particular sodium.
In devices in which the potential between conductivity type zones exceeds more than a few volts, typically more than about 50 volts, measures are taken to inhibit the formation of conduction channels at or near the device surface. Such channels give rise to voltage breakdown and to leakage currents. Field plates are provided in such devices to inhibit accumulation of charge and consequent inversion of the conductivity type at or near the surface of the device. A field plate typically comprises a conductive film formed on top of the dielectric layer, overlying at least a portion of the more lightly doped conductivity type zone for some distance on one side of a PN junction, and is conductively connected to a more heavily doped zone in the body.
The formation of the channels is believed explainable as follows. When a high reverse bias is applied to a PN junction, which may be an N.sup.+ P or a P.sup.+ N junction, under a field plate, minority carriers are injected from the more heavily doped zone to the more lightly doped zone. Ordinarily, no problem arises unless trapping centers are present in the dielectric layer, in which case conductive channels can be formed across the lightly doped zone. This results in undesirably high reverse leakage currents. The silicon dioxide-silicon nitride interface and the silicon nitride layer itself usually contain such trapping centers, particularly when sodium is present.
Thus, it is an object of this invention to avoid the formation of conductive channels under the circumstances described above.